[PrO.GameR]

PHP код:

if(IsPlayerNPC(playerid))
{ 
SetPlayerVirtualWorld(playerid, playerid);
} 


Like that? I don't know how you spawn NPCs but I'm assuming when 1 player connects you connect a NPC for them, right?
if not you should make an array keeping a list of which NPC is in which player's world, when a player connects assign one of the NPCs to the player.